It's pretty obvious that the bolt face for .303 would be different, as well as the extractor. Possibly the feeding flute in left side of the bolt might also be a different shape to accommodate the wider rim and/or the pronounced taper of the .303 case. The cam-track plate attached to the right-hand side of the receiver might be all right if it's for an 8mm gun, but it might be problematic if it's for .30-06.

M